Screen control with cathodes having low electronic affinity

ABSTRACT

A drive system which makes it possible to drive a matrix of picture elements, each including a cathode made of a material with low electron affinity. Each of crossover-point circuits include a switching device associated with a cathode of a picture element and makes it possible, with the aid of memory circuits, to connect the cathode to a current source during a time necessary for the driving of all the rows of the matrix and to regulate the current conduction of the corresponding picture element. Such a drive system may find particular application to electron guns and display screens.

BACKGROUND OF THE INVENTION Discussion of the Background

Materials with negative electron affinity or low electron affinity areknown, which are generally of carbon with diamond structure. Thesematerials have the great advantage of emitting electrons under weakextraction fields (of the order of 10 V/μm). Since it is easy to obtainsuch fields on a planar thin film, it is no longer necessary to createtips in order to fabricate cathodes, and this facilitates thefabrication process. For example, in a tipped cathode it is essential tocontrol the diameter of the holes in the extraction grid to within 0.1μm.

W. Zhu et al. have studied polycrystalline diamond deposits obtained byCVD (chemical vapour deposition) and have shown that the emissiondensity increases significantly with the density of defects which thefilms contain. Certain deposition conditions make it possible to obtainlayers exhibiting, for fields of the order of 30 V/μm, current densitiesof 10 mA/cm², i.e. a sufficient value for fabricating a screen with aluminosity of 300 cd/m². However, the emissive properties of the filmsdo not appear very uniform because they depend greatly on the surfaceroughness (of the order of the grain size ≈5 μm) and the defect density.In field-emission screens whose cathodes are made of polycrystallinematerial, it is therefore found that the display is not uniform.

SUMMARY OF THE INVENTION

The invention makes it possible to solve this problem by proposing tomake the cathodes of an information display screen from alow-electron-affinity material of amorphous or crystalline structurewhich exhibits a smooth surface condition. However, such cathodes cannotemit a strong electron flux (less than 1 mA/cm², about 10⁻⁵ A/cm²). In amatrix screen, for example of 1000×1000 rows, the picture elements arein principle driven row by row. In order to solve the problem of lowpower emitted by each pixel (each cathode), it is proposed to associate,with each cathode, a switching device which sustains the drive of thecathode during a frame time, a frame time being the total time necessaryfor driving all the rows of a screen one after the other. Under theseconditions, it can be assumed that the intensity emitted by a cathodeintegrated over a frame time is virtually equivalent to the power whichwould have been necessary in a row-by-row drive, multiplied by thenumber of rows. In other words, according to the invention, thelow-electron affinity cathodes characterized by a low emission density(<1 mA/cm²) can be used in a display screen so long as they are eachcombined with a drive circuit which sustains the current supply during aframe time, which makes it possible to have a current supply n timessmaller than that which would have been necessary in a row-by-row drive,n being the number of rows of the screen.

The invention therefore relates to a drive system for a screencomprising at least one electron-emission picture element with lowelectron affinity, characterized in that it includes:

a set of cathodes arranged in rows and columns, and driven row by row;

a switching device associated with the cathode of each picture elementand making it possible to connect the said cathode to a current sourceduring a time necessary for the driving of all the rows and to regulatethe current conduction of the corresponding picture element.

BRIEF DESCRIPTION OF THE FIGURES

The various subjects and characteristics of the invention will becomemore clearly apparent from the description below and the appendedfigures, in which:

FIGS. 1a and 1 b represent simplified examples of a cathodic emissiondevice in which the cathode is a material with low electron affinity;

FIG. 2 represents a matrix of devices such as those in FIGS. 1a and 1 b;

FIG. 3 represents a crossover-point drive circuit of a device of thematrix of FIG. 2;

FIG. 4 represents a diagram of the operating times of the circuit ofFIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1a represents a basic structure of the device according to theinvention. This device includes, on a substrate 2, a layer 21, ofmaterial with high electron affinity. On this layer 21, there is atleast one element 1 of material with low electron affinity, calledcathode. In the case of a display device, there is a layer of conductivematerial, called anode 3, facing the cathode at a distance d_(ca) fromthe cathode.

The layer 21 is preferably conductive and makes it possible to drive thecathode electrically. If the substrate exhibits the properties of thelayer 21, the latter may be omitted.

According to the invention, the cathode is made of material deposited inamorphous form so as to exhibit a good surface condition. Itscrystalline structure may optionally be modified by a post-depositiontreatment (heat or laser treatment). This material may, for example, beof carbon with the following structure: a-C:H; a-C:H:N.

FIG. 1b represents an electron microgun. Such a structure is similar, asregards the electron-emission part (cathode), to that in FIG. 1a.However, the anode will be replaced by a target (not shown).Furthermore, an electrode 5′ is provided for focusing the electron beam.This electrode is located above the grid 5 and surrounds theelectron-emission art of the device.

Such devices are arranged in rows and columns in order to permit matrixdrive. FIG. 2 represents such an organization, comprising a matrix ofcathodic emission devices DC1.1 to DCn.m which are connected to rowwires CL1 to CLn and to column wires CC1 to CCm. Drive circuits CDL andCDC make it possible to apply drive potentials to the row wires and tothe drive wires.

Each cathodic emission device is connected to a row wire and to a columnwire via a coincidence circuit or a crossover-point circuit DC1.1 toDCn.m. FIG. 3 represents, for example, the crossover-point circuit CTijconnected to the row wire CLi (i=1 to n) and to the column wire CCj (j=1to n).

Each crossover point of the matrix hence includes a circuit asrepresented in FIG. 3. The circuit includes a first transistor T1ijwhose gate GSij is connected to a row wire CLi and whose source (oremitter) DSij is connected to a column wire CCj. A first capacitor Ctijis connected to the drain (or collector) of the transistor T1ij. Asecond transistor T2ij makes it possible to connect the capacitor Ctij,and more precisely the common point Aij of the capacitor Ctij and of thetransistor T1ij, to a second capacitor Csij. The voltage level of thissecond capacitor Csij makes it possible to control the conduction of athird transistor T3ij which controls the current supply of the cathodeof corresponding crossover point. More precisely, the second transistorT2ij makes it possible to connect the point Aij to the common point Bijof the capacitor and of the gate of the third transistor T3ij. Lastly, afourth transistor T4ij makes it possible to short-circuit the secondcapacitor Csij in order to discharge it. The transistors T2ij and T4ijare driven by drive pulses applied to their gates at specific momentswhich are defined in the diagram of FIG. 4.

The mode of operation of the circuit of FIG. 3 will now be describedwith reference to FIG. 4.

The signals VGS1 to VGSn (represented by the lines VGS1=VGSn) correspondto the drive signals of the rows CL1 to CLn. It can hence is seen that,during a time T which corresponds to a frame time, all the rows havebeen driven one after the other. Attention will be paid, for example, tothe drive signal VGSi of the row CLi. Its period is hence equal to T.

During each row-drive pulse such as VGSi, a column-drive pulse of aparticular value (lying between 0 and 10 V) is applied to each columnwire. From one row-drive pulse to the next, the values of the columnpulses are changed according to the drive operation which it is desiredto perform.

In FIG. 4, only the pulses VDSj sent on the column CCj and, inparticular, to the crossover point of the row i and of the column jrepresented in FIG. 3, have been represented. During the frame time T1,the pulse VDSj1 has, for example, a value of 10 volts. During the frametime T2, the pulse VDSj2 has a value of 5 volts and, during the frametime T3, the pulse VDSj3 has a value of 7 volts.

The effect of the pulse VGSi1 is to turn on the transistor T1ij, whichtransmits the potential VDSj to the point Aij. The capacitor Ctijbecomes charged between this potential and earth, that is to say to apotential of 10 volts in the case of the first pulse VDSj1.

At the end of period T1, a pulse φ1.1 (row φ1) which is produced afterthe last row-drive pulse VGSn of the frame T1, the transistor T2ij isturned on. It should be noted that this signal φ1 is applied to all thetransistors T2ij of the various crossover points of the matrix. In eachcrossover-point circuit, the point such as Aij is connected to the pointBij. The capacitor Csij is hence charged to the potential of Aij. Thepotential of the point Bij turns on the transistor T3ij, and the latterallows a current to flow to the device DCij and hence to the cathode ofthe crossover point to be driven. Following the pulse φ1.1, thetransistors such as T2ij disconnect the points Aij from the points Bij.The current supply of the device DCij is sustained by the transistorT3ij under the control of the capacitor Csij.

After the interruption of the pulse φ1.1, the following frame time T2begins. The column pulse VGSi2 causes the transistor T1ij to conduct.The potential VDSJ2 is transmitted to the point Aij and causes thecapacitor Cti to charge.

Before the following pulse φ1.2, a pulse φ2.1 causes the transistorssuch as T4ij of the various crossover-point circuits to conduct. Therole of these transistors is to earth the points Bij. All the capacitorssuch as Csij of the various crossover points are hence discharged. Thetransistors such as T3ij enter the off state and no longer conductcurrent to the devices such as DCij. Each pulse φ2.1 lasts long enoughto allow the capacitors Csij to discharge. When the pulses φ2.1 cease,the system delivers the following pulse φ1.2 for driving the transistorsT2ij.

As was seen above, the capacitor Ctij of each crossover-point circuithas been charged under the control of the pulses VGSi2 and VDSj2. Theconduction of the transistor T2ij causes the charge of the capacitorCtij to be transferred to the capacitor Csij. The transistor T3ij isturned on again as a function of the voltage level of the capacitorCtij. Operation then continues as has just been described.

It can hence be seen, as represented in FIG. 3, that a crossover-pointcircuit may be regarded as consisting:

of a first memory circuit M1 connected to a row wire and to a columnwire and comprising the transistor T1ij and the capacitor Ctij;

of a second memory circuit M2 comprising the capacitor Csij;

of a transfer circuit CT connecting the memory circuit M1 to the memorycircuit M2 and comprising the transistor T2ij;

of a current-control circuit CCT driven by the memory circuit M2 andcomprising the transistor T3ij;

of a circuit CLEAR for resetting the memory circuit M2 and comprisingthe consistent T4ij.

According to the mode of operation described above, the various rows aredriven successively during a frame time.

Each time a row i is driven, the memories M1 of the row i are loadedwith the data items of the columns. At the end of time of a frame, allthe memories M1 of the matrix are loaded. The transfer circuit CT thenbrings about the transfer of the content of the memories M1 to thememories M2, then isolates the memories M2 from the memories M1. Thememories M2 drive the current-control circuit CT while the data of thefollowing frame time are being loaded into the memories M1. At the endof this following frame, the resetting circuit CLEAR erases the contentof the memories M2, then the transfer circuit CT again brings about thetransfer of the content of the memories M1 to the memories M2. Operationcontinues as described above.

It should be noted that the operation of the system is placed under thecontrol of a central control circuit CCU. The latter drives therow-by-row scanning of the matrix and the sending, for each row-driveoperation, of appropriate potentials on the column wires. The circuitCCU also delivers the signals φ1 and φ2 at the appropriate moments inconformity with the description above, for example according to thetiming diagram of FIG. 4.

The last line of signals of FIG. 4 illustrates the application of thesystem to an electron gun. In such a type of application, the electronbeam emitted by a cathode matrix is directed at a face of a(semiconductor) component to be processed. At a given moment itilluminates one component region, and at a subsequent moment the beam ismoved on the surface of the component and illuminates a neighbouringregion. The last line of FIG. 4 illustrates this movement. At a givenmoment, the beam illuminates a region x1. Next, the beam is moved (forexample by 50 nm), the drive of the matrix is modified and the beamilluminates the region x2. Again, the beam is moved, the drive ismodified, then the beam illuminates the region x3, etc.

What is claimed is:
 1. A screen drive system comprising: a matrixincluding rows and columns of cathodic emission devices that are drivenone full row at a time; and a set of switching devices, each switchingdevice configured to regulate current from a current source to arespective cathodic emission device throughout a time necessary to driveall rows of the matrix; wherein: A) the coincidence circuit includes: 1)a first memory circuit connected to a row wire and to a column wire, andconfigured to store a data item transmitted on the column wire, 2) atransfer circuit configured to controllably transfer the data itemstored in the first memory circuit to a second memory circuit, 3) thesecond memory circuit configured to store the data item transferred bythe transfer circuit, and 4) a current-control circuit configured tocontrol transmission of a defined current to the respective cathodicemission device as a function of the data item stored in the secondmemory circuit; B) the system further comprises a central controlcircuit that is configured to sequentially control scanning of the rowsof the matrix and send the data item on each column each time a row isdriven so that the data item is then stored in the first memory circuit,and to control the transfer circuit of each coincidence circuit totransfer the data item from the first memory circuit to the secondmemory circuit at an end of a frame scan of the matrix.
 2. The system ofclaim 1, wherein each coincidence circuit further includes: a resettingcircuit configured to erase the content of the second memory at the endof a frame and before the transfer circuit transfers the data item fromthe first memory circuit to the second memory circuit.
 3. The system ofclaim 1, wherein each of the first and second memory circuits includes:a capacitor capable of being charged to potential levels correspondingto the data item received on the column wire.
 4. The system of claim 1,wherein: each cathodic emission device includes a cathode that is madeof a conductive material with low electron affinity and of amorphousstructure, whereby a current required from the current source is reducedby a factor of about the number of rows in the matrix as compared tomatrices including cathodic emission devices not using said conductivematerial with low electron affinity.
 5. The system of claim 1, whereineach switching device further includes: a coincidence circuit, connectedto a row wire to receive a row wire potential, and connected to a columnwire to receive a column wire potential; and a connection circuitincluded in the current-control circuit, and configured to connect thecathodic emission device to the current source in response to thecoincidence circuit, and to conduct current of a magnitude correspondingto the column wire potential.
 6. the system of claim 5, wherein eachfirst memory circuit includes: a first capacitor connected to a firstpoint that is between the coincidence circuit and the connectioncircuit.
 7. The system of claim 6, wherein: each second memory circuitincludes a second capacitor connected to a second point that is betweenthe first point and the connection circuit; and the transfer circuit isconfigured to controllably connect the first capacitor at the firstpoint to the second capacitor at the second point.
 8. The system ofclaim 1, further comprising: at least one anode facing the matrix ofcathodic emission devices.
 9. The system of claim 1, wherein: eachcathodic emission device includes a cathode that is made of a materialwith low electron affinity, whereby a current required from the currentsource is reduced by a factor of about the number of rows in the matrixas compared to matrices including cathodic emission devices not usingsaid conductive material with low electron affinity.
 10. A screen drivesystem comprising: a) a matrix including rows and columns of cathodicemission devices that are driven one full row at a time; and b) a set ofswitching devices, each switching device configured to regulate currentfrom a current source to a respective cathodic emission devicethroughout a time necessary to drive all rows of the matrix, whereineach switching device includes: 1) a coincidence circuit; 2) aconnection circuit; 3) a first capacitor connected to a first point thatis between the coincidence circuit and the connection circuit; 4) asecond capacitor connected to a second point that is between the firstpoint and the connection circuit; and 5) a transfer circuit configuredto controllably connect the first capacitor at the first point to thesecond capacitor at the second point.
 11. The system of claim 10,wherein: A) each switching device includes: 1) a first memory circuitthat includes the first capacitor, and that is connected to a row wireand to a column wire, and configured to store a data item transmitted onthe column wire, 2) a second memory circuit that includes the secondcapacitor, and that is configured to store the data item transferred bythe transfer circuit, and 3) a current-control circuit that includes theconnection circuit, and that is configured to control transmission of adefined current to the respective cathodic emission device as a functionof the data item stored in the second memory circuit; and B) the systemfurther comprises a central control circuit that is configured tosequentially control scanning of the rows of the matrix and send thedata item on each column each time a row is driven so that the data itemis then stored in the first memory circuit, and to control the transfercircuit of each switching device to transfer the data item from thefirst memory circuit to the second memory circuit at an end of a framescan of the matrix.
 12. The system of claim 11 wherein each switchingdevice further includes: a resetting circuit configured to erase thecontent of the second memory at the end of a frame and before thetransfer circuit transfers the data item from the first memory circuitto the second memory circuit.
 13. The system of claim 11, wherein: thefirst and second capacitors are configured to be charged to potentiallevels corresponding to the data item received on the column wire. 14.The system of claim 10, wherein: each cathodic emission device includesa cathode that is made of a conductive material with low electronaffinity and of amorphous structure, whereby a current required from thecurrent source is reduced by a factor of about the number of rows in thematrix as compared to matrices including cathodic emission devices notusing said conductive material with low electron affinity.
 15. Thesystem of claim 10, wherein: the coincidence circuit is connected to arow wire to receive a row wire potential, and to a column wire toreceive a column wire potential; and the connection circuit isconfigured to connect the cathodic emission device to the current sourcein response to the coincidence circuit, and to conduct current of amagnitude corresponding to the column wire potential.
 16. The system ofclaim 10, further comprising: at least one anode facing the matrix ofcathodic emission devices.
 17. The system of claim 10, wherein: eachcathodic emission device includes a cathode that is made of a materialwith low electron affinity, whereby a current required from the currentsource is reduced by a factor of about the number of rows in the matrixas compared to matrices including cathodic emission devices not usingsaid conductive material with low electron affinity.